1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer.
2. Related Background Art
Paper or sheet employed as the recording material in the conventional electrophotographic printer or copying machine has principally been so-called exclusive electrophotographic paper prepared with broad-leaf and/or needle-leaf pulp and certain additives such as caolin and rosin. Acidic paper was principally used for this purpose in the past, but neutral paper has recently been increasing in consideration of the storability and environmental issues.
For the neutral paper which has been recently used as the exclusive electrophotographic paper, calcium carbonate CaCO.sub.3 is increasingly used as the additive in the North America and Europe. Such calcium carbonate includes heavy calcium carbonate which is prepared by crushing lime rock or is called chalk originated from ancient organisms, and light calcium carbonate which is prepared by crushing and heating lime rock to obtain CaO and blowing carbon dioxide gas into lime milk prepared by dissolving CaO in water. Such calcium carbonate, being inexpensive and providing excellent whiteness, is suitable for neutral paper and is often mixed, in the paper, up to 25% by weight.
On the other hand, the fixing unit of the printer or the copying machine may employ a pair of rollers as shown in FIGS. 5 and 6. In the fixing unit shown in FIG. 5, a fixing roller 60 and a pressure roller 61 are not given a bias voltage, and a felt member 63 containing silicone oil is maintained in contact therewith.
In the fixing unit shown in FIG. 6, a voltage of negative polarity is applied to the fixing roller, in order to prevent offsetting a toner 68 of negative polarity present on a recording material 67. Also to a pressure roller 61, a diode 65 is connected in order to further prevent the toner offsetting. In addition, the felt member for cleaning the fixing roller 60 is omitted in order to dispense with the trouble of replacement of the felt member. In these drawings, there are also shown a halogen lamp 62 and a lower entrance guide member 66.
However, the paper with a high content of calcium carbonate, when used in the printer or the copying machine utilizing the fixing unit as shown in FIG. 5 or 6, may result in the following drawbacks.
Powdered calcium carbonate is easily charged to positive polarity by friction with a metal or a plastic material. Thus, the calcium carbonate powder present in paper is easily charged to positive polarity by friction between papers when the paper is fed from the paper cassette, or by friction of the paper with a transport guide member or rollers in the transport path.
Also, when the paper is contained in the paper cassette and is fed therefrom, there is generated paper dust by the contact of paper with various rollers such as paper feed roller, transport roller, registration roller etc. or with guide members in the paper transport path, and such paper dust is deposited on the fixing roller. If paper rich in calcium carbonate content is used, calcium carbonate deposited on the fixing roller causes offsetting of the toner charged to negative polarity from the paper onto the fixing roller, since calcium carbonate is charged to positive polarity as described above. The offset toner adheres to the fixing roller, in mixed state with calcium carbonate thereon, and then is transferred onto the surface of the pressure roller of a temperature lower than that of the fixing roller, since calcium carbonate more easily sticks to the pressure roller of a lower temperature of 80.degree. C. to 120.degree. C. than to the fixing roller of a higher temperature of 180.degree. C. to 200.degree. C. Even if such transfer phenomenon takes place by a very small amount for each paper, there is generated a large black flake of toner on the surface of the pressure roller after passing of several tens of thousands of sheets. When such large flake of toner is developed, part of it sticks onto the fixing rollers, thus perturbing the toner image present on the recording material.
Tests with various papers have revealed that such phenomenon is frequently observed when the content of calcium carbonate in paper reaches 10 to 25% by weight.
In the conventional image forming apparatus, it is already known, as disclosed in the Japanese Patent Laid-Open Application No. 7-120993 (cf. FIG. 7), to provide a friction-reducing member such as a Teflon (trade name) adhesive tape and a coated layer of a similar material on the lower bottom face of the developing unit 70 opposed to a pre-transfer guide plate 69.
However such configuration is proposed to prevent "character skip (void)" in the image transfer to a rigid paper such as postcard, by reducing the contact frictional force between the upper face of the transported paper P and the lower face area of the bottom portion 70a of the developing unit 70, thereby preventing the loss of the transported speed of the paper P and maintaining the speed difference between the peripheral speed of the phototsensitive drum and the transported speed of the paper P at a level similar to that in case of ordinary paper. Consequently, the configuration of the image forming apparatus shown in the Japanese Patent Laid-open Application No. 7-120993 is unable to sufficiently suppress the generation of paper dust.